15. (a) Define: (i) signal to noise ratio (SNR) (ii) random, or Gaussian electrical noise. (b) State two sources of random noise. (c) Explain what is meant by repetitive averaging. What must be known about a signal for this to be effective as a noise reduction technique? (d) A certain signal from a sensor is amplified and emerges from the output of the amplifier with an SNR of 10 dB. (i) If the rms signal value at the amplifier output is 2 volts, what is the rms value of the noise component? (ii) Assuming the phase of the signal is known, how many repetitive averages are required to improve this to (i) 20 dB (ii) 30 dB?